As an example of a drying furnace, there is a hot air circulation drying furnace. In this drying furnace, an object is dried by hot air which is circulating. The object is often a combination of a plurality of members and includes thick portions and thin portions. The thick portions have a large heat capacity and thus are regions (hereinafter, referred to as “hard-heating regions”) which are difficult to be warm. On the contrary, the thin portions have a small heat capacity and thus are regions (hereinafter, referred to as “easy-heating regions”) which are easy to be warm. When the object is dried by the hot air circulation drying furnace, a drying operation is continuously carried out until the hard-heating regions are completely dried, despite the fact that the easy-heating regions have been completely dried. Consequently, drying time becomes longer. For this reason, a technology to improve a speed of temperature rising of the hard-heating regions is demanded.
Conventionally, as a technology to improve the speed of temperature rising of the hard-heating regions of the object, drying techniques for locally heating the object have been variously suggested, in addition to a heating by hot air (see Patent Document 1 (FIG. 4), for example).
Patent Document 1 is described with reference to FIG. 11.
As shown in FIG. 11, a drying furnace 100 includes a furnace body 102, a pair of hot air inlets 105, 106, a pair of hot air ejecting ports 109, 111, a pair of hot air suction ports 114, 115 and a pair of hot air outlets 116, 117. The furnace body 102 is formed to surround an object 101. The hot air inlets 105, 106 are respectively provided at a lower portion of a left side wall 103 and a lower portion of a right side wall 104 of the furnace body 102 to introduce hot air therethrough. The hot air ejecting ports 109, 111 are respectively provided at lower headers 107, 108 connected to the hot air inlets 105, 106 to eject the introduced hot air into the furnace body 102. The hot air suction ports 114, 115 are respectively provided at upper headers 112, 113 to suck the hot air in the furnace body 102. The upper headers 112, 113 are respectively provided at an upper inner side of the left side wall 103 and an upper inner side of the right side wall 104. The hot air outlets 116, 117 are respectively connected to the upper headers 112, 113 to exhaust the hot air out of the furnace body 102.
A blower is connected to the hot air outlets 116, 117. A heating device is connected to a discharge side of the blower and the hot air inlets 105, 106 are connected to the heating device. In addition, a pair of left and right heaters 118, 119 is disposed near the bottom of the object 101. These heaters 118, 119 are connected to the blower and the heating device.
In the drying furnace 100, the hot air ejected from the hot air ejecting ports 109, 111 is brought into contact with the object 101 and thus the object 101 is dried. The hot air flows out of the furnace body 102 through the hot air suction ports 114, 115 from the interior of the furnace body 102. The hot air is heated and ejected again from the hot air ejecting ports 109, 111 through the hot air inlets 105, 106. That is, the object 101 is dried by the hot air which is circulating.
Further, if a bottom 121 of the object 101 is thick, the bottom 121 has a large heat capacity and thus is the hard-heating region. Ejection nozzles 122 of the heaters 118, 119 eject the hot air toward the bottom 121 of the object 101. As the hot air is ejected in this way, it is possible to improve the temperature rising speed for the bottom 121 of the object 101.
However, since the hot air ejected from the ejection nozzles 122 of the heaters 118, 119 is spread around the bottom 121 of the object 101 in the drying furnace 100, it is difficult to give a target amount of heat to target regions. If a drying time becomes longer as a measure, the temperature of the bottom 121 which is the hard-heating region is increased, but the easy-heating region of the object 101 is subjected to excessive heat, despite the fact that the temperature of the easy-heating region is already increased. This is undesirable from the viewpoint of energy saving.